1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators and surface acoustic wave filters, and manufacturing methods therefor, and more particularly, to a surface acoustic wave device using a Shear Horizontal type (xe2x80x9cSH-typexe2x80x9d) surface acoustic wave and having a structure for reducing a transversal mode spurious ripple, and a manufacturing method therefor.
2. Description of the Related Art
In surface acoustic wave devices, aluminum or alloys including aluminum as a main component have conventionally been widely used as the electrode material of an interdigital transducer (IDT). At least one IDT is disposed on a piezoelectric substrate and reflectors or reflective end surfaces are disposed at both sides of the area where the IDT is located so as to define a resonator or a longitudinally coupled resonator filter.
In such a surface acoustic wave device, it may be possible that the IDT functions as a waveguide to generate a transversal mode wave, and ripples caused by the transversal mode wave are generated in a pass band. To reduce the ripples caused by the transversal mode wave, various methods have been attempted. Those methods include a method for reducing the intersection width of IDTs and a weighting method.
A surface acoustic wave device has also been proposed in Japanese Unexamined patent Application publication No. Hei-11-298290, in which a quartz substrate is used, an IDT made from a metal or an alloy having tantalum (Ta), which has a larger mass than aluminum (Al), as a main component is disposed on the quartz substrate, and an SH-type surface acoustic wave is used. Since the IDT is made from a metal or an alloy having tantalum, which has a large mass, as a main component, the number of the pairs of the electrode fingers of the IDT is as small as 10 to 20, and thereby the surface acoustic wave device is made compact.
When an electrode material having a large mass-load effect, such as a material having Ta as a main component, is used, the sonic speed obtained at the area where an IDT is located becomes much lower than the sonic speed obtained around the area. Therefore, a waveguide effect is very large at the IDT portion.
Consequently, when a longitudinally coupled resonator filter is produced, ripples caused by a transversal mode wave become complicated and very large, as indicated by arrows X in FIG. 13.
As described above, as methods for removing ripples caused by a transversal mode wave from the pass band of a filter or from the vicinity of a resonant point of a resonator, a method A in which an intersection width is made small and the frequency distance between a basic-mode wave and a transversal mode wave is made large, and a method B in which the intersection width of an IDT is weighted with a cos2 function to eliminate the transversal mode wave have been conventionally attempted.
In the method A, it is necessary to set the intersection width to 10xcex or less, where xcex is the wavelength of a surface acoustic wave. When a quartz substrate and an IDT having 10 to 20 pairs of electrode fingers are used to provide a surface acoustic wave device, the input and output impedance exceeds 2 kxcexa9 and is very high, so that the surface acoustic wave device cannot be used for actual products. Therefore, it is necessary to increase the number of the pairs of electrode fingers to reduce the impedance.
More specifically, whereas the surface acoustic wave device disclosed in the above-described publication uses tantalum, which has a large mass, as a main component to form electrodes and allows the number of pairs in IDTs to be reduced, when the method for reducing the intersection width is used, the number of the pairs of electrode fingers needs to be increased to reduce the input and output impedance. Therefore, the surface acoustic wave device cannot be made compact.
In the method B, weighting itself increases a loss of the surface acoustic wave device. In addition, since weighting reduces the area of an intersection-width portion, the impedance of the surface acoustic wave device becomes very high in the same way as in the method A. Therefore, to reduce the impedance, the intersection width needs to be twice as large as the required length. As a result, the surface acoustic wave device cannot be made compact.
In other words, when either the method A or the method B is used, if ripples caused by a transversal mode wave are to be reduced, the advantage of the surface acoustic wave device in reducing the size of the device as disclosed in the above-described publication is prevented from being achieved.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device which has electrodes made from a material having a larger mass-load effect than aluminum, which is made to be very compact, which has a structure that minimizes and eliminates ripples caused by a transversal mode wave, and which uses an SH-type surface acoustic wave, and a manufacturing method therefor.
According to a preferred embodiment of the present invention, a surface acoustic wave device using an SH-type surface acoustic wave includes a quartz substrate, and at least one interdigital transducer disposed on the quartz substrate and made from electrodes having a larger mass-load effect than that of aluminum, wherein the metallization ratio xe2x80x9cdxe2x80x9d and the normalized film thickness h/xcex of the interdigital transducer are within a range such that a ripple caused by a transversal mode wave is about 0.5 dB or less, where xe2x80x9cxcexxe2x80x9d is the wavelength of the surface acoustic wave and xe2x80x9chxe2x80x9d indicates the film thickness of the electrodes of the at least one interdigital transducer.
In the surface acoustic wave device, the metallization ratio xe2x80x9cdxe2x80x9d and the normalized film thickness h/xcex of the interdigital transducer are preferably controlled so as to be within specific ranges such that the ripple caused by the transversal mode wave is about 0.5 dB or less. Therefore, even in a case in which an IDT made of electrodes having a larger mass-load effect than that of aluminum is used, and the number of the pairs of electrode fingers is reduced to make the device very compact, the ripple caused by the transversal mode wave is effectively suppressed and eliminated. Consequently, a compact surface acoustic wave device using an SH-type surface acoustic wave and having excellent frequency characteristics is provided.
In the surface acoustic wave device, the interdigital transducer may include at least one electrode layer made from a metal having a larger mass than that of aluminum.
In the surface acoustic wave device, the interdigital transducer may be made from a single metal having a larger mass than that of aluminum.
The above-described advantages are also achieved in another preferred embodiment of the present invention which provides a surface acoustic wave device using an SH-type surface acoustic wave, including a quartz substrate, and at least one interdigital transducer disposed on the quartz substrate and made from tantalum, wherein the normalized film thickness h/xcex of the interdigital transducer is within a range from about 0.6d+1.65 to about 0.6d+1.81, where xe2x80x9cdxe2x80x9d indicates the metallization ratio of the interdigital transducer, xe2x80x9cxcexxe2x80x9d indicates the wavelength of the surface acoustic wave, and xe2x80x9chxe2x80x9d indicates the film thickness of the electrodes of the at least one interdigital transducer.
In the surface acoustic wave device according to this preferred embodiment, since at least one interdigital transducer made from tantalum is disposed on the quartz substrate, and the normalized film thickness h/xcex of the interdigital transducer falls in a range from about 0.6d+1.65 to about 0.6d+1.81, where xe2x80x9cdxe2x80x9d indicates the metallization ratio, ripples caused by a transversal mode wave are effectively suppressed. Therefore, even in a case in which at least one IDT made from tantalum is formed, and the number of the pairs of the electrode fingers in the IDT is reduced to make the device very compact, the ripple caused by the transversal mode wave is effectively suppressed and eliminated. Consequently, a compact surface acoustic wave device using an SH-type surface acoustic wave and having excellent frequency characteristics is provided.
The above-described advantages are also achieved in still another preferred embodiment of the present invention that provides a surface acoustic wave device using an SH-type surface acoustic wave, including a quartz substrate, and at least one interdigital transducer disposed on the quartz substrate and made from tungsten, wherein the normalized film thickness h/xcex of the interdigital transducer is within a range from about 0.6d+0.85 to about 0.6d+1.30, where xe2x80x9cdxe2x80x9d indicates the metallization ratio of the interdigital transducer, xe2x80x9cxcexxe2x80x9d indicates the wavelength of the surface acoustic wave, and xe2x80x9chxe2x80x9d indicates the film thickness of the electrodes of the interdigital transducer.
In the surface acoustic wave device, since at least one interdigital transducer made from tungsten is disposed on the quartz substrate, and the normalized film thickness h/xcex of the interdigital transducer is within a range from about 0.6d+0.85 to about 0.6d+1.30, ripples caused by a transversal mode wave are minimized and eliminated even in a case in which the number of the pairs of the electrode fingers in the IDT is reduced to make the device very compact. Consequently, a compact surface acoustic wave device using an SH-type surface acoustic wave and having excellent frequency characteristics is provided.
According to the surface acoustic wave device having the above-described unique structure, the transversal mode ripple is minimized to about 1.5 dB or less.
In the surface acoustic wave device according to various preferred embodiments of the present invention, the normalized film thickness h/xcex may fall in a range from about 0.6d+1.00 to about 0.6d+1.23. In this case, the transversal mode ripple is suppressed to about 0.5 dB or less.
In the surface acoustic wave devices described above, a plurality of the interdigital transducers may be provided to constitute a longitudinally coupled resonator filter. In this case, a compact longitudinally coupled resonator filter having excellent frequency characteristics is provided. In the surface acoustic wave devices described above, longitudinally coupled resonator filters may be connected in a cascade arrangement in at least two stages.
In the surface acoustic wave devices described above, the interdigital transducer may be disposed on the quartz substrate to constitute a one-port surface acoustic wave resonator. In this case, a compact one-port surface acoustic wave resonator having excellent frequency characteristics is provided.
The surface acoustic wave devices described above may be configured such that a plurality of the interdigital transducers is disposed on the quartz substrate, wherein each of the interdigital transducers constitutes a one-port surface acoustic wave resonator, and the plurality of the interdigital transducers are connected to constitute a ladder-type filter on the quartz substrate.
The surface acoustic wave devices described above may also be configured such that a plurality of the interdigital transducers are disposed on the quartz substrate, wherein each of the interdigital transducers constitutes a one-port surface acoustic wave resonator, and the plurality of the interdigital transducers are connected to constitute a lattice-type filter on the quartz substrate.
In the above two cases, compact ladder-type and lattice-type filters having excellent frequency characteristics are provided.
The surface acoustic wave devices described above are widely used for surface-acoustic resonators and surface acoustic wave filters. The foregoing advantages may also be achieved through the provision of a communication device using one of the surface acoustic wave devices according to preferred embodiments of the present invention described above.
The above-described advantages are achieved in yet another preferred embodiment of the present invention through the provision of a manufacturing method for a surface acoustic wave device using an SH-type surface acoustic wave, including the steps of preparing a quartz substrate, forming a metal film having a larger mass-load effect than that of aluminum on the quartz substrate, and patterning the metal film by reactive ion etching or by a lift-off process such that the metallization ratio xe2x80x9cdxe2x80x9d and the normalized film thickness h/xcex of the interdigital transducer, which reduce a spurious transversal mode ripple to about 1.5 dB or less are satisfied, to form at least one interdigital transducer, where xe2x80x9cdxe2x80x9d indicates the metallization ratio of the interdigital transducer, xe2x80x9cxcexxe2x80x9d indicates the wavelength of a surface acoustic wave, and xe2x80x9chxe2x80x9d indicates the film thickness of the interdigital transducer.
In the manufacturing method for a surface acoustic wave device using an SH-type surface acoustic wave according to a preferred embodiment of the present invention, a metal film having a larger mass-load effect than aluminum is disposed on the quartz substrate, and patterning is applied to the metal film by reactive ion etching or by a lift-off process such that the metallization ratio xe2x80x9cdxe2x80x9d and the normalized film thickness h/xcex which decrease a transversal mode ripple to about 1.0 dB or less are satisfied, to form at least one interdigital transducer. Therefore, even when the pair of the electrode fingers is reduced to make the device compact, the transversal mode ripple is suppressed in the device. In addition, since patterning is performed by the reactive ion etching or the lift-off process, an IDT satisfying the foregoing normalized film thickness h/xcex is always formed.
The manufacturing method for a surface acoustic wave device using an SH-type surface acoustic wave, according to a preferred embodiment described above, may be configured such that the metal film is made from tantalum, and patterning is performed by the reactive ion etching or by the lift-off process such that the normalized film thickness h/xcex is within a range from about 0.6d+1.50 to about 0.65d+1.87, preferably in a range from about 0.6d+1.65 to about 0.6d+1.81, to form at least one interdigital transducer.
The manufacturing method for a surface acoustic wave device using an SH-type surface acoustic wave according to the preferred embodiment described above may be configured such that the metal film is made from tungsten, and patterning is performed by the reactive ion etching or by the lift-off process such that the normalized film thickness h/xcex falls in a range from about 0.6d+0.85 to about 0.6d+1.30, preferably in a range from about 0.6d+1.00 to about 0.6d+1.23, to form at least one interdigital transducer.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments below with reference to the attached drawings.